This project has as its long-term aim the elucidation of the role of enzymatic repair mechanisms, which act on damaged DNA, in the induction of mutations by ultraviolet light. The main strategy of the project has been to compare ultraviolet mutagenesis in pairs of strains, otherwise isogenic, which differ in a single mutation affecting their ability to perform a known repair function. Previous work has led us to conclude that ultraviolet light-induced mutations originate as errors in the repair of single-strand gaps in DNA opposite unexcised pyrimidine dimers. Our future work will be largely directed at obtaining information about the nature of the error-prone repair mechanism responsible for these mutations. One hypothesis now being investigated intensively is that the error-prone repair system is inducible, rather than constitutive, and that it is produced in the bacterial cell only when certain kinds of damage are present in the DNA.